The ability to choose a specific light output setting of an illumination system is desirable in many applications ranging from indicator lights, displays, optical communication systems, and lighting applications in general. For example, reducing the light intensity or turning off a portion or all of the lights in an illumination system is a very effective method to reduce power consumption. It is common practice, for example, to dim or de-energize the light-emitting elements when an office space is unoccupied or there is a change in ambient illumination due to daylight ingress in order to save energy. It is also common practice to dim or de-energize the light-emitting elements when the use requirements of an occupied space changes, such as, for example, when video projectors are used in an office space. In addition, it has recently become practical through the advent of solid-state lighting to change the color temperature and, more generally, the chromaticity of the light-emitting elements to mimic the changes in color temperature of natural daylight and so synchronize the circadian rhythms of night-shift workers.
In some applications, the light level may be controlled manually, semi-automatically or automatically through various controls and sensors. Illumination systems may also be controlled to provide multiple colors, or to change colors. For example, a multi-color light-emitting diode (LED) illumination system can transition through dozens of brightness and color combinations; they are commonly used in architectural, restaurant, commercial, mood lighting, decoration, parties, or special lighting environments. Being able to select an individual color, brightness level, and/or the light intensity distribution of an illumination system allows users to save energy as well as to match the light output with the environmental situation and design requirements.
The current-voltage characteristics (“I-V curve”) of semiconductor LEDs are such that the forward voltage VF across the device remains relatively constant (e.g., within about 0.5V) within the device's normal operating range (FIG. 1). Consequently, the luminous flux output of the device can be controlled by varying the current flow (the “drive current”) through the device by means of a constant-voltage power supply and a variable resistance connected in series with the LED, or by means of a constant-current power supply connected directly to the LED.
Currently, achieving a target illumination level from (e.g., dimming) LEDs may be accomplished by controlling the forward current flowing through the LEDs. Two common methods are analog dimming and pulse modulation dimming. Typically, analog dimming uses a variable resistor or a current regulator circuit to dynamically adjust current flowing through the LEDs and thus change the brightness thereof. This approach has a number of disadvantages. First, the current-voltage characteristics of individual semiconductor LEDs may vary, even within a single manufacturing batch. This may result in two LEDs generating different luminous flux outputs for the same drive current, particularly when the drive current approaches the “knee” of the I-V curve (FIG. 1). This may be problematic when the LEDs are electrically connected in series and mounted as an array on a common circuit board, particularly when the LEDs are directly visible to the viewer. Second, when the current is varied, not only is the light output power of the LED changed, but so, undesirably, are the color characteristics. This is problematic for general illumination applications, which typically mandate strict limits on any variations in lamp chromaticity. Depending on the circuitry involved, the efficiency and power factor may vary with different dimming levels, which is also undesirable. Third, blue- and green-emitting indium-gallium-nitride (InGaN) LEDs exhibit a secondary emission mechanism that tends to generate yellow light at low drive currents. This limits the dynamic range of drive currents for InGaN LEDs to approximately 100:1 before the change in perceived chromaticity become unacceptable. For general illumination applications, this dynamic range limitation is exacerbated by the nonlinear response of the human visual system, which perceives changes in perceived brightness according to the square root of light source intensity. Hence, while a 50:1 change in drive current may result in a 50:1 change in light source intensity using analog dimming, the change in perceived brightness is only 7:1. Architectural lighting dimming systems often require a greater dynamic range, which makes analog dimming unsuitable for such applications.
Additionally, when series-connected strings of LEDs are dimmed in this way, the method can fail due to the manufacturing variability in the electrical and optical characteristics of LEDs. For example, as the current is reduced, some LEDs turn off before others and some are dim when others are still quite bright. Finally, the use of analog dimming technology tends to increase the overall system power consumption since the analog dimming driver is always active.
Pulse modulation techniques (such as pulse width modulation (PWM) pulse code modulation (PCM) and pulse position modulation (PPM)) dimming techniques utilize a digitally modulated pulse to switch the LEDs on and off at a high frequency (ranging from about 300 Hz to over 100 kHz); the human visual system is typically incapable of perceiving such rapid changes for switching frequencies above 150 Hz, and so perceives the light source intensity as being the average on-time of the digitally switched drive current (FIG. 2). The longer the “on” periods are relative to the “off” periods, the brighter the LEDs will appear to the observer. In this approach the current level is fixed; it could be fixed at any value, but is often fixed at the maximum recommended current for the device, or at a value that provides an acceptable compromise between light output and efficacy). This approach is generally called pulse width modulation (PWM) and is frequently used to dim LED illumination systems.
The advantage of digital dimming in comparison to analog dimming is that the problems related to low drive current are eliminated. However, digital dimming control systems suffer from their own disadvantages. First, they require more complex circuitry than those used for analog dimming, which results in more expensive systems. This is especially true where the digital dimming controller must be capable of interfacing with a phase-cut dimmer control switch designed for incandescent lamp dimming, as additional circuitry is required to translate the AC phase information to the modulated current. It is difficult to design and expensive to manufacture digital dimming control systems that do not exhibit flicker and hysteresis at low light level settings when interfaced with phase-cut dimmer control switches.
Second, efficiency and power factor are often a function of the dimming level, with reduced efficiency and power factor typically occurring at low dimming levels. Third, PWM systems can generate high-frequency electrical noise that can interfere with or disrupt other electronic systems. Without careful design and expensive shielding, such noise may be transmitted into the AC power line and/or emitted as electromagnetic radiation that may potentially exceed allowable limits on radio frequency interference. This electrical interence may interfere with or disrupt other electronic systems such as power line modems and RF-enabled devices.
There is a need, therefore, for solutions that provide dimming control for LED illumination systems to achieve high efficiency and high power factor over the full range of dimmer settings, and providing freedom from undesirable chromaticity shifts and electrical noise and as well as permitting control of illumination characteristics such as color and light distribution.